In the case of image reconstruction are used on various occasions so-called attenuation corrections are in nuclear medicine imaging, for example in positron emission tomography (PET). In the case of PET imaging, light quanta emitted from annihilation of a positron and an electron in opposite directions are registered in the ideal case by a detector pair. This is done by measuring two events within a defined coincidence interval. In this case, the probability that these two light quanta follow their path without disturbance and largely rectilinearly depends, inter alia, on the absorption behavior of the material located in the path. Strongly absorbing materials such as, for example, bones, plastic and metals lead to a reduction in the detection rate.
This means, in turn, that without correction mechanisms the emission regions that lie “behind” absorbing regions are displayed in the image with an excessively low intensity (that is to say false quantification) or even in a distorted fashion (if there is an asymmetric absorption geometry). This is countered by the so-called attenuation correction (AC).
In the course of the attenuation correction, a spatial distribution of the absorption behavior is determined on the basis of models or a preceding measurement. In the simplest case, such a model can be spherical, such as, for example, in the case of head pictures. The measurements can be carried out with the aid of different modalities or of the nuclear medicine detectors themselves, for example with the aid of a PET detector and an additional rotating x-ray source. What is decisive is that the measurement can be used to say how the spatial distribution of the absorbing materials appears in the examination region.
In the case of combination of modalities (particularly hybrid modalities), with the aid of which both the taking of magnetic resonance tomography pictures and taking of nuclear medicine pictures are possible, the attenuation correction or an associated attenuation correction map is produced by and large with the aid of magnetic resonance tomography data.
However, there is a problem with the attenuation correction required for image reconstruction that the patient and, if appropriate, further constituents in the image region that are assigned to the patient or are arranged on the body of the latter or in the body of the patient can change their position during movement operations of the patient. During such movement operations, for example, periodic movement operations such as breathing or movement of the heart or else movements of limbs and the like, the attenuation correction map originally made no longer reproduces the current attenuation, as a result of which errors can occur when reconstructing the nuclear medicine images in the previously customary way with the aid of the initial attenuation correction map.